This invention relates to a device for filtering and separating liquid mixtures that have been stratified and more particularly to a serum separator for filtration and separation of serum from blood samples. In the Grover U.S. Pat. No. 3,693,804, and the Haldopolous U.S. Pat. No. 3,832,141, assigned to the assignee of the present application, serum separators are disclosed for performing this function. In these serum separators, the sample is collected in a test tube and an elongated sampling tube is provided to fit within the test tube. In the lower end of the sampling tube is a piston head which makes a sliding seal with the sidewalls of the test tube. A filter is mounted in a passageway extending through the piston head to the interior of the sampling tube. As the sampling tube and piston head are pushed into the test tube, fluid will pass through the filter into the interior of the sampling tube leaving the residue in the bottom of the test tube. Then when the sampling tube is withdrawn, the filtrate, which will be serum, will be withdrawn with the sampling tube. The piston head in both the Haldopolous patent and Grover patent is provided with an annular lip which makes the sealing engagement with the sidewalls of the test tube. In the Grover patent, this seal is broken to remove the sample by a sideways motion of the sampling tube within the test tube. In the apparatus of the Haldopolous patent, the lip is provided on its upper edge with irregularities. When the sampling tube with the piston head is removed from the test tube, the lip reverses its position and the upper side of the lip engages the sidewall of the test tube. As a result, the irregularities in the upper surface of the lip break the seal between the lip and the sidewall.
As an improvement in the above described serum separators of the prior art, it has been proposed to provide a one-way valve in the filter head between the filter and the interior of the sampling tube so as to prevent backflow from the sampling tube down through the filter when the sampling tube is removed from the test tube. The one-way valve also serves to isolate the serum from the residue of the blood sample so as to provide an ion exchange barrier after the filtering operation. The system of the present invention provides such a serum separator with an improved version of the one-way valve. The separator of the present invention is easily manufactured consistently in a form that achieves prevention of backflow and complete isolation of the serum from the residue so that an effective ion barrier is achieved.